The invention relates to resonant cavities and electronic devices and is directed more particularly to a design for an optically tunable cavity for an electronic device yielding high-Q operation and thus, constant high-efficiency bandwidth.
In a typical klystron, radio frequency (RF) signals are amplified by converting the kinetic energy in a direct current (DC) electron beam into radio frequency power. The beam of electrons is accelerated by high-voltage electrodes (the voltage being typically in the tens of kilovolts). This beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, the natural frequency of the cavity to produce a voltage which acts on the electron beam. The electric field causes the electrons in the beam to “bunch.” Electrons that pass through the cavity during the period when the RF input energy creates an electric field opposing the electron in the beam are accelerated and later electrons are slowed, causing the previously continuous electron beam to form “bunches” at the input frequency. To reinforce the “bunching,” a klystron may contain or consist of additional “buncher” cavities. The RF current now carried by the electron beam produces an RF magnetic field, and this may in turn excite a voltage across the gap of subsequent resonant cavities. In an output cavity, the developed RF energy is coupled out. The spent electron beam, with reduced energy, is captured in a collector. Tuning a conventional klystron with respect to a specified RF frequency may be a delicate procedure and may require several steps and considerable time.
Typically, vacuum electronic devices are narrowband by nature. In recent years, dual cavity klystrons have achieved about 8 percent experimental electronic bandwidth by utilizing a dual gap out filter scheme. The trade-off for this increase in bandwidth is loss of gain, and thus, efficiency. Including solid state amplifiers, to date, no known device of reasonable bandwidth (for example, greater than 10 percent) yields more than about 40 percent direct current-radio frequency (DC-RF) efficiency. Narrowband (less than 2 percent) devices exist with high efficiencies (greater than 88 percent), but such oscillators are inherently narrowband and thus cannot, with past and current techniques, be constructed to yield large bandwidths.
Piezoelectric devices have been used to variably tune resonant cavities. However, these devices require additional electrical inputs, outputs and may generate interference, requiring additional shielding. Some of these piezoelectric devices require other adaptive measures. For example, U.S. Pat. No. 4,737,738, issued to Perring, describes a resonant cavity having movable planar members where a piezoelectric crystal is attached to one of the movable planar members.
There is a need for an optically tunable cavity for a vacuum electronic device that achieves both high bandwidth and high efficiency.